Surface components of the human pathogen Neisseria gonorrhoeae have been shown to change dramatically during the course of gonorrheal disease in male volunteers. The Opa family of outer-membrane proteins change expression states as a result of a unique translational switching mechanism involving the insertion or deletion of discrete numbers of pentameric coding repeat elements (CR) within the segment of the gene coding for the signal-peptide of the Opa pre-protein. The direct, tandem repeat elements have the capacity to form an intramolecular triplex (H-DNA) in response to negative supercoiling torsion. The phase variation frequencies of repeated genetic elements which form H-DNA are significantly lower than elements with the same repetitive nature which are unable to form H-DNA. The enzymes responsible for generation of negative supercoiling (gyrA/B) have been cloned and are being purified to determine their effect on the transcription of opa and the relationship between transcription and H-DNA formation. A number of Opa proteins have been constitutively expressed as beta-lactamase-Opa fusions in E. coli and have been shown to segregate to the outer- membrane in a conformation resembling that seen in N. gonorrhoeae. Recombinant Opa+ bacteria mimic the interactions seen between human neutrophils and gonococcal strains expressing the analogous Opa. Recombinant strains also show an ability to adhere to certain eukaryotic cell lines in in vitro binding assays. One particular Opa fusion protein confers upon the recombinant E. coli strain the ability to adhere to and be taken up by Chang conjunctival epithelial cells. A Type III restriction modification system has been cloned from N. gonorrhoeae MS11. This system may be unique in its expression since at this time it appears that expression of the upstream methylase enzyme is translationally regulated by a series of direct tandem repeats in a manner similar to that found for the Opa system (although this has not been confirmed at this point). If found to operate as a control mechanism for expression this system will provide an interesting contrast to the Opa system with respect to the mechanisms employed for translational regulation by "illegitimate" forms of recombination.